Method and apparatus for ionic discharge coating



Jan. 27, 194s. J, E', JOHNSON 2,434,931

METHOD AND APPARATUS FOR IONIC DISCHARGE COATING /N VEA/ro@ J B. JOHNSONBV w@ ATTORNEY Jan. 27, 1948. I J, B, JQHNSON 2,434,931

METHOD AND APPARATUS FOR IONIC DISCHARGE COATING Filed Deo. 1, 1944 2sheets-sheet 2 y 4o 62 6/ /2 l43 42 4/ f4 s i l I I 6/ /NVENTOR J B.JOHNS ON ATTORNEY Patented Jan. 27, 1948 "METHOD AND APPARATUS FOR IONICDISCH'ARGE COATING John B. .iohnso'mMaplewooi N. J., assignorlto .BellTelephone Laboratories, Incorporated, ,New ,York,.N.azcorporatoniof NewYork Application jDecember 1, 1944, Serial' No. 566,063

.fg-Claims. f1

Thi'sinvention Vrelates to method-'and apparatus for ionic `dischargecoating and `more specifically to .methods of andr means forA'generating 'positive ionbeams Vand lfor utilizing said 'ideama-forexample, iin the `formation` of 'targets for lectron `beams in electroncameratubes.

.'One well-known type of `electronfcamera tube is 'called'.th'eiconoscopei vIn the'usualiorm of J this tube, there 'is provided 'amosaic target com- 'prising a metal .backing or signalplateyaninsulatinglayer onithe signal plate 'and ra discontinuous layer vofphotosensitized metallic -globules y or Vparticles-on1the insulatinglayer. Radiations from fan object are applied to ythis `target .and itis scanned with a beamo'f electrons to pro- 'ducefazsignalrcurrent'inearesistor which is foon- `nected to the l'signal plate. IIn A-awelliknown -ymethod offmaki-ng mosaic targetsfthe insulating layer whichfusually is 'of lmicafor glass is coated with a metallic Alayer-ofA'silver -andfby a heating 'process vthe'silver is'broken up intodiscontinuous particles which vare oxidized 'andsensitized 'withv'caesium to form caesiumeoxidesilver globules. In screens made by thisprocess,theparticle`s 'are fof irregular shape anddistribut-ion, f thusleading Ato non-uniformity -of photos'ensit'ivity over the *surfaceofthe mosaic. "This inventioniihone yof l"its primary aspects `relates-to 'mosaic targets VAin which this non-uniformity is avoided 'orgreatly reduced.

It is'ian object of'thisinventionito provide novel rrietl'iodsyoffandlmean's for making mosaicltargets for'electrons,the-particles ofthe mosaic being of 4substantially "uniform shape, `size =and`ldistribution.

In fa I`oop'en'ding application 'of the same `linventor, .SerialNo,'lf84966, 'Tfiled eAprl 29, 194B, thereis :described various methods"and means "of 'utili'zingib'eans of positive'ion's o'i metallic "ma-'terial .in '..the formation off mosaic "tar-gets. The rions may @be :offaim-etal which is lphotosensiti-ve, fsuchias Ianfaikali or :alkalineearth metal,`orthey :may .be of metal .wlfiich .is fnotfappreciably pho-.tosensitive .In i latter instance, the 'rnetailic.spotsformedibyith'efmethod described'in thecopending. applicationare'photosensitize'd as inthe usual ".iconoscope *target technique. Infa #sec- -fond fscopending application 'of the Esame inventor,SerialfNo. .55iigillledliec'emberi, 31914442'. beam foi fp'ositively:charged fhydrogen 'I ions is produced byion-optical means andutilizedto k'scan-'lia' 'metallic oxideior other suitablenone-conducting.reduciblejayer'to vproduce ionsfotmetalfin a".seavoiiinsulation. :Oxides of aluminum 'or :one :of the solidimetalsfromfgroup IIA in :the `.periodic vtable Yaio are suitable'for`this'purposel y"The ions of v'metal vare then photosensitized asinithe.usual iconofscope technique.

'-In accordance with vthe present invention, :an

unmodulated oxygen 'positive ion `beam rproduced sensitivity increasedby rsensitizing it after the -oxygenion treatment With a suitable alkali'metal A'such as caesium. A very uniform pattern is produced by thismethod.

More 'specically, in accordance with an exemplary form of the invention,an oxygen positive ion beam is generated in aside tube connected to thecamera tube. This side tube comprises an evacuated container venclosing`a rst electrode member and one or more additional `electrode members.Oxygen is introduced into the side tube and a'posit'i've ion dischargeis set upwithin the container which discharge is formed into a beamof'ions 'by placing the additional electrode members kat appropriatenegative potentials with respect to the rst electrode member. This beamof oxygen positiveions is directedinto `the tube containing thetarget'to be 'formed'and .there uti-` lized as described above.

Oxygen `gas is present in the space adjacent the target, Vand tends toslowly oxidize the whole target surface. A metal is chosen which has anormally low rat-e of oxidation. Such a metal'is aluminum or vone yof'the solid metals of group "II in the periodic table. Th'eionic oxygenof the scanning beam, however, has a much greater abilityfto combinewith the metal, and oxidation Awillbe-'greatly accelerated at thecontact of the `'beam with "the surface.

The 'density of normal oxygen near the target is'kept low by means of apump applied to tubulation 10 vso as to inhibit'the general oxidationiover'the surface.

AIna specific method of operation, the oxygen 'positive ion beam iscaused 'to scan 'the `target (by electromagnetic or Aelectrostaticmeans) vin'parallel lines, the vbeam being cut off in alternateiinescanning periods so that oxide strips sepa- :rated by metallicstrips are produced. Many seconds or even `minutes maybe necessarytorscan the complete .target in orderto oxidize lalternate 'stripsof'the'target to therequired thickness. The

beam is 'then caused to "scan alternate .strips gin fia :directioneo.degreesff from ythe former direction so as to produce spaced oxidizedlines perpendicular to those produced during the first scanning. Afterthis scanning, conducting islands are left between oxide lines. A veryuniform pattern of square metal islands is produced. In an alternativemethod, the target is formed by repeated scanning, that is, the spacedoxide strips are lbuilt up or increased in density on successivescannings until the complete oxide lines are built up. The direction ofscanning is then changed as before and the oxide lines at 90 degreeswith respect to the first series of lines are built up by successivescannings.

As the photosensitized target loses its sensitivity when exposed to air,the gun system for the electron beam is preferably mounted in the tubebefore the ionizing process. The gun system for the positive ion beam ismounted in a side tube which is joined to the main tube. Oxygen isadmitted into the side tube and the positive ion beam is formed by theelecti'odes therein and the ion beam is caused to scan the target in themain tube. After the target is oxidized by the ion beam, the connectionbetween the main tube and the side tube is broken, the main tube issealed off and pumped and the target is subjected to any heat treatmentswhich are necessary.

The invention will be more readily understood by referring to thefollowing description taken in connection with the accompanying drawingsforming a part thereof in which:

Fig. 1 is a schematic diagram to illustrate a process, in accordancewith this invention, of making a target for an electron camera tube bymeans of an oxidizing ion beam;

Fig. 2 is an enlarged view of a portion of the target for the ion beam;and

Fig. 3 is a modification of the arrangement shown in Fig. l.

Referring more specifically to the drawings, Fig. 1 shows an arrangementfor forming an oxygen positive ion beam and for utilizing this beam toform a mosaic target for electrons, the target being contained in anelectron camera tube attached during the process to the tube wherein theion beam is generated. The main tube I containing the target I4 is shownconnected to the auxiliary tube II by means of a narrow neck I2 which isadapted to be cut and the ends sealed after the mosaic target in themain tube has been formed. The main tube IU comprises an evacuatedcontainer I3 enclosing the target I4 which is to have a mosaic surfaceformed thereon, an electron gun I5 suitable for generating and focussinga beam of electrons upon the target, and two sets of deiiecting elementscomprising the magnetic coils I5, I6 and I1, I1. The electron guncomprises, for example, a cathode 20, a cathode heater 2I, a controlelectrode 22, a rst anode 23, a second anode 24, and a conductingcoating 25 which is preferably placed at the sam potential as the secondanode 24.

The target I4 is placed in position within the camera tube so that itcan be struck by the electron beam generated and focussed by the gun I5and also opposite an opening 26 in the conducting coating 25 through-which radiations from an object are adapted to be applied to the targetwhen the tube is finally completed and operating. During the formationof the mosaic surface on the target I4, however, the electron gun I5 isnot operative nor are radiations applied to the target. The target I4preferably comprises a metallic backing plate 3U and a thin layer ofmica 4 or glass 3| thereon held to the metal backing plate 30 by anysuitable means such as by screws or clamps (not shown). The insulatingcoating 3| has applied thereto a thin layer of a readily oxidizable (byionic oxygen) metal, such as, for example, aluminum. Other suitablemetals are those in group II of the periodic table. The layer 32 is thentreated by means of the ion beam generated in the side tube I I in amanner which will be described more fully below.

The tube II which is connected to the tube I3 f by means of the neck I2at a point thereof somewhere between the region of the gun I5 and thedeilecting coils I6, I6 and I1, I1 comprises an evacuated container 40enclosing therein a first electrode 4I, a second electrode 42 and athird electrode 43. The electrode 4I is shown as a fiat plate while theelectrodes 42 and 43 are shown as cylinders, the cylinder 43 being oflarger diameter than the cylinder 42. Electrode member 4I is connectedto the electrode 42 through a source 44 and a make-and-break contactmember 45, the purpose of which will be described below. The positivepole of the source 44 is connected to the electrode member 4I and thenegative pole thereof is connected through the member 45 to theelectrode member 42. The electrode 43 is placed at a negative potentialwith respect to the member 42 by means of a source 41 while the backingplate 3i) of the target I4 in the tube I3 is placed at a negativepotential with respect to the electrode member 43 by means of the source48. Within a side tube 49, oxygen is produced and applied to theevacuated container 40.

The manner in which the apparatus shown in Fig. 1 operates to producemetallic particles 52 (see Fig. 2) surrounded on all sides by oxidestrips 53 and 54 is as follows: The tube II is connected to the tube I0by means of the glass neck I2. If it is desired to photosensitize thetarget while it is in the camera tube, the target cannot be exposed tothe air without losing its photosensitivity. For this reason theelectron gun I5 is mounted in the tube I D and a conducting coating 25is applied t0 the portion of the walls in the enclosure by well-knownmeans before the tube I I is joined to the tube I0. Moreover the targetstructure comprising the signal plate 3l) and the insulating layer 3Iand the coating of suitable metal such as aluminum is placed in positionbefore these two tubes are joined together. A yoke holding the magneticcoils I6, I5 and I1, I1 is placed around the tube I0 (in some cases itmay be desirable to also perform this step before the tube is connectedto the tube II) and magnetic deflecting current is applied to thesecoils, which current is preferably of saw-toothed wave form. Nopotentials are applied to the members of the electron gun I5 as theelectron beam does not enter into the process of making the mosaictarget coating. A magnetic coil schematically represented bythe circle50 is placed in posit-ion so that the beam of oxygen positive ions whenformed in the side tube I I is directed toward the target I4. Beforeadmitting oxygen into the tube II all gases are removed from this tubeby means of side tube 5I, heat being applied during this step ifdesired. The side tube is sealed off after all occluded gases areremoved from both the tube II and the tube I0. With the electrodeelements 4I, 42 and 43 connected as shown in Fig. l of the drawing, theelement 42 is at a negative potential with respect to the electrodemember 4I inasmuch as the top half (as shown in Fig. l) of the contactmember arrancar l-isiofc'onductingv material while the' bottom half inAofi` noni-conducting material; Arv streamv of positive' ions is thusformed" between membersi 4! `and 42'. This stream. is focussed intoabeam by means' of the electrostatic field between` these' members andbetween the members 42 andi 43, which are at. appropriate negativepotentials. with respect'A to the member' 41| and thus. accelerate thepositive ions and produce a: focussing. action on the' positive ions ina mannerv similar" to that produced on electrons by cylinders'vrconnected at positive potentials with respect to a cathode. Thispositive ion beam is focussed' to' a small' cross-section at the surfaceoff. the. target' I4, the' be'am being bent' by th'e constant currentyapplied to the magnetic coil 5U.. The' source. 48 can be omitted but inmany cases' it? may be desirable to` have an additional accelerating eldbetween the' electrode ele:- ments 43r and" thetarget' lll. Saw-toothedcurrent waves of linearV slope are applied to the' coils t6, t5: and Il,|11. The frequency of these waves can: of; the; order, respectively, ofthe line scanning; and. frame' scanning frequencies 1 used in the.'usual television system; The member l5` is rotated. at.' such. a speed'that it.' cuts: off the: ion beam'. during' alternate line scanningintervals sof asi to leave' a space between horizontal lines 53 as.shown in. Fig. 2. The: frequency of rotations of the member 45' is, forexample, equal to one-half the line scanningy frequency of the waveappliedtoloneofv the coils I6, I6 or l1, il. Each line 5:3r may b'e of awidth whichis much less than thewidth' of an elemental area asdetermined by'v the sizeof electron beam in the gun I5 or may besubstantially equal in widthv to that of elemental areav and an adjacentinsulating area. or.' substantiallyr as` wide as an elemental area. if'the. pathof the electron beam is carefully controlled so'that itgoes'over lines of sensitiz'ed" metal islands and skips, by one means'or anothen the; lines of insulation only. By means of. this. method,4eaclr horizontal line 53 is formed by cumulative oxidation ofV thealuminum inthe layer.Y 3.2. Itmay' take many seconds or even severalminutesv before a satisfactory thickness of. insulation. producedl bythe oxidation of aluminum of the member 32 by the oxygen positive ionibeam isn reached.

In al1-alternative way of operating the arrangementfshown inFig. 1-,stepped saw-toothed waves of frame scanning frequency are appliedv toone o! the.: sets ofy coils I 6 le and l1, H, while a saw-toothed waveform of line scanning frequency'isapplied to' theY other sety of coils.These waves.` are preferably of such low frequency' that one completescanning of the target is sufficient to build'. up` ea'ch oxide line' 53to the` desired thickness. Because of the waves beingv stepped, theion'. beam. remains. onV oner line until the end thereof is' thenalmostinstantaneously jimped. tot the next. element. line' 53,. leaving, a.blank' .spacci between these two lines; as shown in: Fig..21 Such astepped wave canv be; pro.- duced by a potentiometer having' a numberof' taps. witharotating member connecting. each of these taps in` turnto' the` magneticy coils. An:- other' potentiometerl comprising aslidingY contact on afcoil can bie'-v used to produce the? saw-toothedwave- (the upper wave shown atv the right. of Fig. 1);. The twopotentiometers can be geared together tol maintain the two waves in.propertime relation: with. e'a'ch other but thisy may be' doneelectrically' as` in. ordinary' television scanning'. Ituwill be clearthats the frequency of the. saw'- toothed. wave. applied to: one: setyof coils the frequency'ofthestepped wave applied to: thefother setoffcoilshave thesameratio asthe frequencies of 'the two saw-toothed waves.described above. When such current' waves are used' in the; coil, the'member 45 can be. removed` and` replaced by a directconneiction betweenvthe` negative pole of the source. 44 and the. positive pole. ofA thesource 4.1. or' allowed to remain stationary in the conductingposition..

After' the'. horizontal oxide lines 53 are formed on'. the metalliclayer: 312, the direction of scanning. is shifted. by 9,0 degrees by anysuitable means such' as, for example, by applying the line scanningfrequency' current to the. coil formerly producing the frame scanningand the frame scanning frequency current to the coil formerly producingtheline frequency scanning. The vertical lines 54. (shown in Fig. 2)arevthus produced in the manner corresponding to the-horizontal line 53.This leaves metal islands 52 ,having their surfaces slightly oxidized bythe generali pressure of oxygen and completely surrounded. by oxidelines. These. metal islands. 52 are thus of rectangular shape and thepattern is.veryuniform'.y

After the oxide lines 53 and 5t` are formed, leaving the metal islands52, the neck. I2 is sealed off and'- the tube Hll baked out andevacuated so that' all of the elements-off the tube are thoroughlydegassed. The tube is then allowed to cool to room temperature. A knownamount of caesiurn is then admitted into the bulb by flashing a caesiumpill. The pill is flashed in a side tube or in the tube il. so thatcaesium vapor passes into or is present in the tube to photosensitizeythe surfacel oxidized metallic particles 52. A description of a suitableprocess of photosensitizing with a caesium pill and the composition ofsuch a pill may be found in British Patent 381,606, to George. R.Stilwell and Charles H.

Prescott, Jr., complete accepted October 10, 1932'. The tube is thensealed off.` I

The operation of the mosaic screen in the cathode ray tube Ii! issimilar to that of the well'- known iconoscope tube described in anarticle entitled The iconoscope, by V. K. Zworykin in the January 1934Proceedings of the Institute of Radio Engineers, pages 16 to 32,inclusive, and an article by the same author in the July 1936v RCAReview, page 60, entitled Iconoscope and kinescopes in television.

The tube Il, as mentioned above, can be used over and over again for.the preparation of targets n storagetype. tubes, it being merelynecessary to connect it to each tube in turn,r go through the processdescribed above, and then break theconnection between the two.

Fig. 3 shows. apparatus. ofY somewhat different form fromy that shown.in Fig. 1 for utilizing, an oxygen positive ion beam to form a mosaictarget for electrons.` In. the arrangement shownin Fig, 3,'.def1ectngplates Eil and. El., lil. are used to replacethe magnetic. coils l5, leand. Il. l1 of Fig.. 1 which are used to cause. the oxygen positive ion,beam tor scan the mosaic target I4.. The magnetic bending field 5t isnot required in the arrangement of Fig. 3 due to the. factr that thetube H' is joined tothe tube i0 at such an angle thatthe axis of the ionbeam in its undeected position'. strikes the center. of the target 1.4..Each of. the elements of the apparatus which. is com mon to. the.structures shown in Figs. 1 and. 3 has been given the same referencecharacter in both figures. The oxygen positive'.v ion beam is; formed inthe tube Il in the manner described above in connection with Fig. 1 andthe photosensitlve mosaic is formed on the target I4 as described .abovewith the exception that the scanning is caused by means of voltage wavesapplied to the deecting plates E0, 6e and 6|, 6l instead of currentwaves passed through the magnetic coils iB, I6 and l1, l?. After theformation of the oxide surfaces 53 and 54, thus leaving metallicparticles 52 as metal islands, the neck l2 is sealed ofi' and tube leevacuated and allowed to cool. The metal particles 52 are thenphotosensitized as in the process described above. The deflecting platesGS, 60 and 6i, 6l are left within the tube l0. The process performed bythe apparatus of Fig. 3 in some respects is preferable to that of Fig. 1inasmuch as it is somewhat difiicult to bend an ion beam byelectromagnetic means due to the large mass of the ion as compared withthat of the electron whereas electrostatic deiiection may be obtainedwith neld strengths of the order of those commonly used to deect an electron beam. Another advantage is simplification` the magnetic bending eldor an electrostatic equivalent thereof not being required.

As the metal used as the material of the layer 32 has to be subsequentlyphotosensitized, the mosaic pattern can be applied in a separate tube orreceptacle and then exposed to the air before being inserted in thecathode ray transmitter tube. The transmitter tube in this modiedarrange ment has means for admitting the sensitizing metallic vapor buthas no positive ion gun associated with it in any stage of the process.1n this arrangement, the target is preferably placed at right angles tothe beam.

Alternatively, a tube such as tube H of Fig. 3 can be connected to thetube l0 by means of a rubber hose or similar connection 62. After thetarget has been formed with its pattern of crossed-lines 53 and 5G,leaving many metallic islands 52, air can be admitted, the connection 62broken and the opening sealed up. In this alternative arrangement,handling of the target after the formation of the pattern thereon isreduced. The tube ii) is then degassed and sensitized in any knownmanner.

While the invention in its primary aspects relates to processes of andmeans for forming a mosaic target for use with an electron beam, it willbe appreciated that the invention in its broader aspects is not limitedto producing targets as the coating may be provided for some otherpurpose for which coatings are used. It

will be understood also that various modifications can be made in thespecific embodiments described above without departing from theprinciples upon which the invention is based. For example, whilescanning by the ion beam of every other line of the target has beendisclosed it is obvious that the main criterion is that a strip ofunscanned target is left between repeated line scannings thereof. Such astrip can be as wide as the ion beam or greater than or less than thiswidth. Preferably the strip should be as small as possible to provide aslarge an active area as possible.

What is claimed is:

1. The combination with a container, of means 'for introducing oxygentherein, spaced electrodes within said container between which saidoxygen is present, a source of potential having its terminals connectedto said electrodes respectively whereby oxygen positive ions move to oneof said electrodes and electrons move to the other of said electrodes,an aperture in said electrode toward which said positive ions move andthrough which some of them pass, and a target for receiving said oxygenpositive ions, said target comprising an element of metallic materialupon which said positive ions impinge to oxidize the portions of saidtarget struck thereby.

2. The combination with a container, of means for introducing oxygentherein, spaced electrodes Within said container between which saidoxygen is present, a source of potential having its terminals connectedto said electrodes respectively whereby oxygen positive ions move to oneof said electrodes and electrons move to the other of said electrodes,an aperture in said electrode toward which said positive ions move andthrough which some of them pass, and a target for receiving said oxygenpositive ions, said target comprising elements of insulating materialhaving a coating of an oxidizable metal facing said beam and a backingof a conducting material electrically connected to said aperturedelectrode.

3. The combination with a container, of means for introducing oxygentherein, spaced electrodes within said container between which saidoxygen is present, a source of potential having its terminals connectedto said electrodes respectively whereby oxygen positive ions move to oneof said electrodes and electrons move to the other of said electrodes,an aperture in said electrode toward which said positive ions move andthrough which some of them pass, a target for receiving said oxygenpositive ions, said target comprising elements of insulating materialhaving a coating of an cxidizable metal facing said beam and a backingof a conducting material electrically connected to'said aperturedelectrode, and means for causing said beam to scan said target wherebyportions of the metallic layer facing said beam are oxidized inaprogressive manner.

4. The combination with a container, of means for introducing oxygentherein, spaced electrodes within said container between which saidoxygen is present, a source of potential having its terminals connectedto said electrodes respectively whereby oxygen positive ions move to oneof said electrodes and electrons move to the other of said electrodes,an aperture in said electrode toward which said positive ions move andthrough which some of them pass, a target for receiving said oxygenpositive ions, said target comprising elements of insulating materialhaving a coating of an oxidizable metal facing said beam and a backingof a conducting material electrically connected to said aperturedelectrode, and means for causing said beam to scan spaced lines of saidtarget and then to scan spaced lines of said target in a direction atdegrees with respect to the iirst scanning.

5. The method of metallic coating which comprises ionizing oxygen in anelectric field formed between two electrodes in an evacuated container,focussing the positive ions formed by said ionization into a beam whichis highly concentrated and of relatively small cross-sectional area, andcausing said beam to scan a two-dimensional portion of a metallic targetsurface comprising a metal from the group consisting of the solid metalsoi group II in the periodic table and aluminum.

6. The method of metallic coating which comprises ionizing oxygen in anelectric eld formed between two electrodes in an evacuated container,focussing the positive ions formed by said ionization into a beam whichis highly concentrated and of relatively small cross-sectional area,andcausing said beam to scan in a manner to produce a uniform oxidepattern thereon a two-dimensional portion of a metallic target surfacecomprising-a metal from the group consisting of the solid mete als ofgroup II in the periodic table and aluminum.

7. The method of metallic coating which comprises ionizing oxygen in anelectric eld formed between two electrodes in an evacuated container,focussing the positive ions formed bysaid ioniza: tion into a beam whichis highly concentrated and of relatively small cross-sectional area,causing said beam to scan in a manner to produce a uniform oxide patternthereon a two-dimensional portion of a metallic target surfacecomprising a metal from the group consisting of the solid metals ofgroup II in the periodic table and aluminum, and photosensitizing themetallic portions of said pattern.

8. The method of metallic coating which comprising ionizing oxygen in anelectric eld formed between two electrodes in an evacuated container,focussing the positive ions formed by said ionization into a beam whichis highly concentrated and of relatively small cross-sectional area, andcausing said beam to scan a two-dimensional portion of a metallic targetsurface in spaced crossed lines to produce a pattern of metallic islandsbetween oxide lines, the metal being from the group of the solid metalsof group II of the periodic table and aluminum.

9. The method of making a mosaic target for an electron beam whichcomprises ionizing oxygen in an electric iield formed between twoelectrodes in an evacuated container, focussing the positive ions formedby said ionization into a beam which is highly concentrated and ofrelatively small cross-sectional area, causing said beam to scan atwo-dimensional portion of a metallic target surface in spaced crossedlines to produce a pattern of metallic islands between oxide lines, themetal being from the group consisting of the solid metals of group II ofthe periodic table and aluminum, and photosensitizing the metallicislands.

JOHN B. JOHNSON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,178,233 Klatzow Oct. 31, 19392,239,642 Burkhardt et al. Apr. 22, 1941 2,157,478 Burkhardt et al. May9, 1939

